log_services: Add download of post code log entries

- Add a GET method /redfish/v1/Systems/system/LogServices/PostCodes
  /Entries/<str>/attachment/, Get the attribute value through the
  getPostCodes method and encode it as base64, and send it off.

- This allows the use to offload error logs for analysis and further
  parsing if needed. An http header of "Accept:
  application/octet-stream" or the default "*/*" is expected.

Tested:
- Ran Redfish validator.

- pldmtool raw --data 0x80 0x3F 0xC 0x0A 0x00 0x00 0x00 0x00 0x00 0x07 0x00 0x00 0x00 0x48 0x00 0x00 0x00 0x02 0x00 0x00 0x01 0x00 0x00 0x00 0x48 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x53 0x54 0x41 0x4e 0x44 0x42 0x59 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20 0x20

$curl -k https://127.0.0.1:2443/redfish/v1/Systems/system/LogServices/PostCodes/Entries/B1-1/attachment/


output:
AgAAAQAAAEgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFNUQU5EQlkgICAgICAgICAgICAgICAgICAgICAgICAg

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I74dd6c1dc2d3dfb7908f7741e0d9e7825c1df816